A MEMS is a device which acquires and processes information and performs operations. A sensor in the MEMS can receive external information such as pressure, location, speed, acceleration, magnetic field, temperature or humidity, and convert the obtained external information into electrical signals to be processed in the system. An example of a MEMS device may include a temperature sensor, a pressure sensor, a humidity sensor and the like.
The cost performance of a MEMS pressure sensor can be greatly improved by reduced size, increased precision and utilization of a process that is compatible with a fabrication process of an integrated circuit chip. Currently, MEMS pressure sensors include a piezoresistive type pressure sensor and a capacitive type pressure sensor. In piezoresistive pressure sensor, a Wheatstone resistance bridge composed of a high-precision silicon resistor strain gage serves as a force-electric conversion measurement circuit according to a principle that the silicon resistor can be changed under a stress (pressure). Therefore, the piezoresistive pressure sensor has a high measuring accuracy and a low power consumption.
In a conventional method for fabricating a MEMS piezoresistive pressure sensor, firstly, multiple resistors or Wheatstone resistance bridges are formed in a silicon substrate by an ion implantation or diffusion process; then a region of the silicon substrate in which the resistors are arranged are etched with wet or dry process to form a sensing thin film; and finally, a pressure reference cavity is formed at the back of the sensing thin film by a sealing process, and a pressure sensor chip is formed after being packaged. The sensing thin film generates a deformation and a stress under an external pressure, and a resistance value of the resistor or resistance bridge is changed. Under a voltage bias, the above-described change of the resistance value is converted into an electrical signal, and the electrical signal being amplified by a signal processing circuit serves as an output signal.
In addition, the electrical signal is transmitted between the pressure sensor chip and the signal processing circuit, so that the electrical signal output by the pressure sensor chip is processed. The pressure sensor chip and the signal processing circuit chip are packaged in a system to form a MEMS piezoresistive pressure sensor.
In the existing methods for fabricating a MEMS pressure sensor, processes for fabricating the pressure sensor chip and the signal processing circuit are different, and it is difficult to achieve a monolithic integration. Moreover, in a case that an integrated circuit and a pressure sensor are fabricated on a single substrate, the existence of pressure sensor creates difficulty in making, changing and improving the integrated circuit, and the integrated circuit on the same substrate makes it difficult to fabricate a small pressure sensor. Therefore, a process for fabricating an integrated circuit and a pressure sensor on a single chip is complicated and a device formed by the current process has a large size, thereby increasing its fabrication cost.
In a case that the pressure sensor and the circuit are integrated on a single substrate, if the pressure sensor is fabricated before the circuit is fabricated, a process for fabricating the pressure sensor often affects the substrate and causes a difficulty in fabricating the integrated circuit, thereby reducing production yield; alternatively, if the integrated circuit is fabricated before the pressure sensor is fabricated, the integrated circuit may limit greatly the choice of material of the pressure sensor and the temperature in a process for fabricating the pressure sensor, thereby decreasing the performance of the pressure sensor.
Hence, there is an urgent need for a method and a structure for effectively integrating a pressure sensor and an integrated circuit.